Structure for transporting, commissioning and decommissioning of a deck of a fixed oil platform and method for implementing the structure

ABSTRACT

A structure for the transport, installation and dismantlement of a deck ( 2 ) of a fixed oil platform ( 1 ) for maritime use, the structure including a floating U-shaped hull ( 11 ) fitted with at least three lifting legs ( 12 ) adapted so as to rest upon the bottom of the sea, each lifting leg ( 12 ) being associated with mechanical displacement units ( 20 ), and a shuttle ( 30 ) which can be displaced along the lifting legs ( 12 ) by the hull ( 11 ) and which is applied to the lower surface of the deck ( 2 ) in a support position for the deck, the shuttle ( 30 ) being provided with locks ( 36, 37 ) for locking on the lifting legs ( 12 ). A method for the transport, installation and dismantlement of a deck of a fixed oil platform is also disclosed.

PRIORITY CLAIM

This is a U.S. national stage of application No. PCT/FR03/00723, filedon 6 Mar. 2003.

Priority is claimed on that application and on the followingapplication:

Country: France, Application No. 02/03637, Filed 22 Mar. 2002.

The PCT International application was published in the French language.

BACKGROUND OF THE INVENTION

The present invention relates to a structure for the transport,commissioning and decommissioning of a deck of an offshore fixed oilproduction platform. Moreover, it relates to a method for the transport,commissioning and decommissioning of a deck of a fixed oil platform.

A known approach to oil production is to install above an oil field afixed oil platform with a deck carrying production equipment andaccommodation premises, in particular. A column anchored to the seabedsupports the deck.

To date, there are two major methods for the transport, commissioningand decommissioning of a fixed oil platform deck.

The first method involves using barge-mounted lifting cranes fortransfering the oil platform deck from the transport vessel to asuperstructure comprising a supporting column. Although this method hasbeen the most widely used to date, it has limitations.

The first limitation is effectively the capacity of the lifting cranes,which may require the deck to be fabricated in several units, therebysignificantly increasing the fabrication cost of the oil platform deckand the cost of commissioning and decommissioning said deck.

The second limitation lies in the fact that this method imposes the needfor a relatively long favorable time window to be able to perform thisoperation at sea under satisfactory conditions.

Without incurring a considerable increase in cost, this method isdifficult to implement in areas in which time windows are relativelyshort, for example in the North Sea.

The second method involves commissioning the oil platform deck in asingle unit by causing it to float over the supporting column. The deckis then positioned on this supporting column either by aballasting/deballasting system or by a mechanical system.

In the case of a ballasting system, the oil platform deck is supportedeither by a floating support comprising, for example, a barge orpontoons or a U-shaped floating support, or by using a structureassociated with this floating support.

In cases in which the superstructure can be ballasted or deballasted,deballasting of the floating support and ballasting of thesuperstructure is a known approach used to decommission an oil platformdeck. The decommissioning operation can be performed relatively quicklybecause the superstructure offers a large ballasting capacity. In casesin which the superstructure is anchored to the seabed, only thedeballasting capacity of the floating support can be used. The operationprogresses slowly because this capacity is limited.

Systems using ballasting or deballasting have drawbacks due mainly tothe fact that they require a complex structure in terms of chambers orpumps and very accurate control of chamber filling and emptying tomaintain floating support stability during the operation. Operationspeed depends on the ballasting and deballasting capacity of thesefloating support chambers, which is generally rather low inasmuch as theoperation speed is often limited, especially when the superstructure isanchored to the seabed. Furthermore, maritime conditions must befavorable in order to perform such an operation in satisfactoryconditions.

An alternative to the ballasting/deballasting system is to use amechanical system to raise or lower the oil platform deck. These systemsenable an oil platform deck commissioning or decommissioning operationto be performed faster than the abovementioned systems.

For this purpose, a known approach involves a system including twobarges that support the oil platform deck using two swivelingstructures. Furthermore, a cable and winch system is used to ensuresystem stability and control the raising and lowering of the oilplatform deck.

Barge clearance is controlled by actuating these winches, therebyallowing the deck to be raised or lowered. However, the stability ofthis type of mechanical system is very insecure and it is frequentlyincompatible in relation to open sea use.

Another mechanical system comprises a rack and pinion system for raisingor lowering the oil platform deck.

In general, mechanical systems used to date for commissioning anddecommissioning an oil platform deck are faster than ballasting ordeballasting systems, but they are dependent on maritime conditions,which makes them difficult to use in areas in which favorable timewindows are relatively short.

SUMMARY OF THE INVENTION

The object of the invention is to overcome these drawbacks by proposinga structure for the transport, commissioning or decommissioning of afixed oil platform deck that has been designed to facilitate differentoperations, whilst achieving a significant saving of time, avoiding anyrisk of polluting the environment and increasing the safety of personnelin charge of performing the various operations.

For this purpose, the subject of the invention is a structure for thetransport, commissioning and decommissioning of a deck of an offshorefixed oil production platform, characterized in that it comprises:

-   -   a U-shaped floating hull fitted with at least three hull lifting        legs, adapted so as to rest upon the seabed, each leg being        associated with mechanical displacement means, and    -   a shuttle which can be displaced along said lifting legs by the        hull and is intended to be applied to the lower surface of the        deck in a support position for said deck, said shuttle being        provided with means for locking on the lifting legs.

Depending on the specific implementation methods:

-   -   mechanical displacement means comprise, on the one hand, two        opposing plates supported by the corresponding lifting leg and        each featuring on both its lateral faces a set of teeth and, on        the other hand, at least two opposing units supported by the        hull and each made up of a pinion driven rotationally and        operating in conjunction with one of the sets of teeth,    -   the shuttle is made up of at least three independent units, each        associated with one leg through guidance means and featuring        lateral stabilization devices for the deck in its support        position,    -   each shuttle unit includes a vertical branch, whose top section        features guidance and locking means for positioning the unit on        the hull and a horizontal branch for supporting the deck,    -   the guidance means are formed by two opposing flanges, each        determining, in conjunction with the vertical branch, a housing        for one plate on the corresponding leg,    -   the locking means comprise at least one counterlocking rack,        which can be displaced by at least one actuation device between        a retracted position and a locked position engaging one of the        sets of teeth on the corresponding leg,    -   the stabilization devices include at least one hydraulic or        pneumatic horizontal displacement cylinder for each shuttle        unit,    -   the hull features rails for sliding the deck horizontally        between a position supported on the shuttle units and a storage        position outside the structure.

A further subject of the invention is a method for decommissioning andtransporting a fixed oil platform deck from a production site to a deckdisassembly quay, characterized in that it comprises the followingstages:

-   -   positioning beneath the deck a transport structure featuring a        U-shaped floating hull fitted with at least three hull lifting        legs and a shuttle which can be displaced along these lifting        legs by said hull,    -   applying the lifting legs to the seabed,    -   raising the hull and the shuttle to bring said shuttle into        contact with the deck,    -   separating the deck from its supporting column,    -   lifting the deck by means of the hull and the shuttle,    -   locking the shuttle onto the structure lifting legs and        laterally stabilizing the deck on the shuttle,    -   lowering the hull to bring it into floatation,    -   continuing hull lowering to cause the structure lifting legs to        rise by reaction,    -   displacing the deck supporting structure to release said deck        from the supporting column,    -   applying the lifting legs to the seabed and raising the hull to        bring it into contact with the shuttle,    -   unlocking the shuttle from the structure lifting legs,    -   lowering the hull supporting the shuttle and the deck to bring        it into floatation,    -   raising the structure lifting legs,    -   floating the deck supporting structure as far as the disassembly        quay,    -   applying the lifting legs to the seabed to stabilize the hull,    -   releasing the deck from the structure,    -   ensuring the link between the hull surface and the quay, and    -   offloading the oil platform deck onto the quay.

A further object of the invention is a method for transporting from aquay to a production site and commissioning a fixed oil platform deck,characterized in that it comprises the following stages:

-   -   floating near to a quay a transport structure featuring a        U-shaped floating hull fitted with at least three hull lifting        legs and a shuttle, which can be displaced along these legs by        said hull,    -   applying the lifting legs to the seabed to stabilize the hull,    -   ensuring the link between the hull surface and the quay,    -   transfering the deck from the quay to the structure shuttle and        laterally stabilizing the deck on this shuttle,    -   raising the lifting legs,

floating the structure until it is near to the production site,

-   -   applying the lifting legs to the seabed,    -   lifting the deck by means of the hull and the shuttle,    -   locking the shuttle onto the structure lifting legs and lowering        the hull to bring it into floatation,    -   raising the structure lifting legs,    -   positioning the deck above the oil platform supporting column        and applying the lifting legs to the seabed,    -   raising the hull to bring it into contact with the shuttle and        unlocking said shuttle,    -   lowering the hull, the shuttle and the deck to place this deck        on the supporting column,    -   fixing the deck to the supporting column,    -   lowering the hull and the shuttle to bring said hull into        floatation,    -   raising the structure lifting legs, and    -   removing the structure from the production site.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood on reading the followingdescription, provided only as an example and made in reference to theappended drawings, in which:

FIG. 1 is a diagrammatic elevation view of a fixed oil platform inproduction position,

FIG. 2 is a diagrammatic perspective view of a structure for transport,commissioning and decommissioning according to the invention,

FIG. 3 is a partial vertical sectional view of mechanical means fordisplacing the structure,

FIG. 4 is a sectional view along line 4-4 in FIG. 3,

FIG. 5 is a diagrammatic perspective view of a structure shuttle unitaccording to the invention,

FIG. 6 is a diagrammatic vertical sectional view of locking means for ashuttle unit,

FIGS. 7A to 7J are diagrams showing the various stages of the methodaccording to the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

A fixed oil platform designated generally by reference 1 and including adeck 2, featuring customary production equipment and accommodationpremises, is diagrammatically represented in FIG. 1. This deck 2 ismounted on a supporting column 3, whose base is anchored to the seabed 4by anchorage devices 5.

A structure designated by general reference 10 and represented in FIG. 2is used to ensure the transport of the deck 2 of the fixed oil platform1 from a production site to a disassembly quay or vice versa, as well asthe commissioning and decommissioning of said deck 2.

General structural dimensions and proportions between the various unitsmaking up this structure 10 have not necessarily been respected in FIG.2, to simplify understanding of the drawing.

In general, the structure 10 includes a U-shaped floating hull 11 fittedwith lifting legs 12 for this hull 11, adapted so as to rest upon theseabed. The hull 11 features two lateral sections 11 a and a linksection 11 b connecting the two lateral sections 11 a.

In the construction example represented in FIG. 2, the hull 11 is fittedwith four lifting legs 12 arranged in pairs on each lateral branch 11 aof said hull 11.

In an alternative embodiment, the hull 11 may be fitted with threelifting legs 12 arranged in a triangle, one leg 12 being located on eachlateral section 11 a and one leg 12 being located on the link section 11b.

The bottom end of each leg 12 terminates with a shoe 13 designed to bearon the seabed 4.

In this construction method, each leg 12 has a triangular section asrepresented in FIG. 2. These legs 12 may also have a square or circularsection.

Each leg 12 is made up of three chords 14 interlinked by a lattice ofsteel beams 15.

Each leg 12 is associated with mechanical displacement means designatedby general reference 20 (FIGS. 3 and 4).

The mechanical displacement means 20 for each leg 12 are located insidea supporting framework 16, also called a “jack-house” by specialists,which itself is supported by the hull 11.

As represented in FIGS. 3 and 4, mechanical displacement means 20 foreach leg 12 comprise, on the one hand, two opposing plates 21, eachsupported by a chord 14 of the corresponding leg 12 and each featuringon both its lateral faces a set of teeth 22 forming a double rack onboth chords 14. Mechanical displacement means 20 also comprise severalunits 25 arranged on either side of each plate 21 according to itsheight. Each unit 25 includes a geared motor unit 26 driving a pinion27, which engages with a set of teeth 22 on the corresponding plate 21.In the construction method represented in FIGS. 3 and 4, both sets ofteeth 22 on each plate 21 are associated with eight pinions 27, eachdriven rotationally by a geared motor unit 26.

The structure 10 also features a shuttle designated by general reference30, which can be displaced along the legs 12 by the hull 11 and isintended to be applied to the bottom face of deck 2 of oil platform 1,as will be seen later.

The shuttle 30 is made up of independent units 31, whose numbercorresponds to the number of legs 12 of the structure 10, as representedin FIG. 2. Thus, in the construction example represented in the figures,the shuttle 30 is made up of four independent units 31, each associatedwith one lifting leg 12.

In an alternative embodiment, the shuttle 30 may also comprise a singleunit in the form of a frame supported by lifting legs 12 usingmechanical displacement means 20.

In reference to FIGS. 3 to 6, a description will be given of oneindependent unit 31 of the shuttle 30, the other independent units 31being identical.

The unit 31 includes a vertical branch 32 for positioning the unit on alateral face of the supporting framework 16 of the hull 11 and ahorizontal branch 33 for supporting the deck 2 during transport. Thevertical branch 32 features guidance means along the corresponding leg12 and means of locking onto the leg 12.

Two opposing flanges 34 located in the top section of the verticalbranch 32 and extending parallel to the horizontal branch 33 of the unit31 form guidance means. Each flange 34 determines, in conjunction withsaid vertical branch 32, a housing 35 for one plate 21 on thecorresponding leg 12. Each unit 31 is thereby guided along thecorresponding leg 12 by plates 21.

As represented by FIG. 6, means of locking each unit 31 are formed by atleast one counterlocking rack 36 and, preferably, by at least onecounterlocking rack 36 for each plate 21. The counterlocking rack 36 canbe displaced by at least one actuation device 37 and preferably by twoactuation devices 37 composed of, for example, hydraulic or pneumaticcylinders to displace the counterlocking rack 36 between a retractedposition and a locked position engaging one of the sets of teeth 22 onthe corresponding leg 12. The combination formed by the counterlockingrack 36 and actuation devices 37 is carried by the unit 31 usingappropriate fixing devices, which are not represented.

Furthermore, each unit 31 of the shuttle 30 is fitted with devicesensuring lateral stabilization of the deck 2 in its supported position.These devices are formed by at least one hydraulic or pneumatichorizontal displacement cylinder 38 located, for example, in thevertical branch 32 of said unit 31.

As represented, in particular in FIGS. 4 and 5, each unit 31 is providedwith two slideways 39, each extending vertically along the lateral edgeof the vertical branch 32. Each slideway 39 operates in conjunction witha vertical slideway 40, fixed to the framework 16 at each lifting leg12. Opposing faces of the slideways 39 and 40 are provided with an antifriction lining made of “Teflon”, for example.

Finally, the top face of the horizontal branch 33 of each unit 31features elastic bearing devices 41 for the deck 2 of the oil platform1. These elastic devices 41 comprise, for example, elastomer blocks orinflatable cushions or any other suitable system.

As represented in FIG. 2, the top face of the hull 11 is fitted withrails 42 extending in the same direction as the lateral sections 11 a ofsaid hull 11, which allow the deck 2 to slide horizontally, after it hasbeen placed on said hull 11, between its position supported on the units31 of the shuttle 30 and a storage position outside the structure 11,for example on a port quay, to allow safe disassembly of the deck 2 forpersonnel or vice versa, in other words transfer of the deck 2 from aquay onto the units 31 of the shuttle 30 for transport of the deck 2 tothe production site.

Finally, the open part of hull 11 features a door 43 formed by, forexample, two gates 44 which can be moved from an open position forpositioning the structure 10 beneath the deck 2 to a closed positionduring transfer of said deck 2 by the structure 10.

Transport of the deck 2 by the structure 10 between the production siteand a port quay is performed in the following way.

First and foremost, as represented by FIG. 7A, the structure 10 isfloated with the gates 44 in an open position beneath the deck 2 of theoil platform 1 by positioning the supporting column 3 for the deck 2 inthe U-shaped space created between the lateral sections 11 a of the hull11. During this positioning, the lifting legs 12 are in a retractedposition such that the shoes 13 are located beneath the hull 11. Theunits 31 of the shuttle 30 are applied against the hull 11, asrepresented in particular by FIG. 2.

Geared motor units 26 are then actuated to rotationally drive thepinions 27, which engage with sets of teeth 22 on plates 21 in eachlifting leg 12 to bring the shoes 13 into contact with the seabed 4.

As soon as the shoes 13 contact the seabed 4, the hull 11 moves upwardalong the lifting legs 12 under the effect of the rotational driving ofthe pinions 27, which engage with sets of teeth 22 on plates 21 in eachlifting leg 12. By moving upward, the hull 11 lifts the shuttle 30 toapply elastic devices 41 of each unit 31 against the bottom face of thedeck 2. Driving of the pinions 27 is stopped and the counterlockingracks 36 are actuated by the cylinders 37 to make these counterlockingracks 36 engage with the adjacent teeth 22 to lock each unit 31 of theshuttle 30 onto the lifting legs 12, as represented in FIG. 7C.

The supporting column 3 is then separated from the deck 2, for exampleby cutting its connecting rods, and the combination comprising hull 11,shuttle 30 and deck 2 is lifted to separate the deck 2 from thesupporting column 3, which remains in a vertical position, asrepresented in FIG. 7C.

After these different operations, the hull 11 is lowered into floatationby reverse driving of the pinions 27, which engage with sets of teeth 22(FIG. 7D). The pinions 27 continue to be rotationally driven, whichcauses the lifting legs 12 to rise by reaction due to the downwardthrust of the hull 11 into the water (FIG. 7E). This allows the hull 11,supporting the deck 2 through the shuttle 30, to be removed from theproduction site, where the supporting column 3 remains in place, asshown in FIG. 7F.

The lifting legs 12 are then relowered to bring the shoes 13 to bear onthe seabed 4 and the hull 11 is raised by rotational driving of thepinions 27 to bring the hull 11 into contact with the units 31 of theshuttle 30 (FIG. 7G). These units 31 of the shuttle 30 are unlocked byreturning the counterlocking racks 36 to their retracted position usingcylinders 37. The combination comprising hull 11, shuttle 30 and deck 2is lowered to bring the hull 11 into floatation, as illustrated in FIG.7H.

During displacements of the shuttle 30 units 31 with respect to the hull11, these units 31 are guided by flanges 34 on the vertical branch 32and by the slideways 39 and 40.

During transport, lateral stabilization of the deck 2 is ensured by thecylinders 38 on each unit 31 of the shuttle 30, which are in contactwith the lateral faces of the deck 2.

The hull 11 supporting the deck 2 is floated (FIG. 7I) to a port quay(FIG. 7J) and the lifting legs 12 are applied to the seabed to stabilizethis hull 11. The cylinders 38 are retracted and a linkspan 6 is placedbetween the hull 11 and the quay to ensure continuity between thesurface of this hull 11 and said quay. The deck 2 is then offloaded ontothe quay, for example by sliding it on rails 42.

The deck 2 is transported by the structure 10 from the port quay to theproduction site by performing reverse operations which are mentioned inbrief hereinbelow.

Firstly, the transport structure 10 is floated near to the quay and thelifting legs 12 are applied to the seabed to stabilize the hull 11. Thelink between the hull 11 surface and the quay is ensured by the linkspan6 and the deck 2 is transferred from the quay onto the shuttle 30 units31 by making it slide on the rails 42.

The deck 2 is then laterally stabilized on the units 31 of the shuttle30 using cylinders 38 on each unit 31, the linkspan 6 is removed and thelifting legs 12 are raised.

The structure 10 supporting the deck 2 is floated near to the productionsite and the lifting legs 12 are applied to the seabed such that thedeck 2 is lifted using the hull 11 and the shuttle 30.

The units 31 of the shuttle 30 are locked onto lifting legs 12 by meansof counterlocking racks 36, which are actuated by the cylinders 37 suchthat these counterlocking racks 36 engage with adjacent teeth 22,thereby locking each unit 31 of the shuttle 30 onto the lifting legs 12.

The hull 11 is brought into floatation and the legs 12 of the structure11 are raised. The deck 2 is positioned above the supporting column 3 ofthe oil platform 1 and the lifting legs 12 of the structure 10 are againapplied to the seabed. The hull 11 is lifted to bring it into contactwith the units 31 of the shuttle 30 and these units 31 are unlocked,then the hull 11, the shuttle 30 and the deck 2 are lowered to placethis deck 2 on the supporting column 3.

The deck 2 is fixed to the supporting column 3, the hull 11 and theshuttle 30 are lowered to bring said hull 11 into floatation, then thelifting legs 12 are raised.

The structure 10 is then removed from the production site.

The structure according to the invention has the advantage of being ableto transport directly the whole oil platform deck from the productionsite to a fixed site, where its disassembly can be undertaken safely andwithout risk of polluting the marine environment or vice versa, from afixed site to a production site, where the deck is mounted on itssupporting column.

Furthermore, the different transfer and transport stages can beperformed without any ballasting operation, thereby achievingconsiderable time saving, which is significant in regions in whichatmospheric conditions change very quickly.

Finally, operations involving the transfer of the deck from thetransport structure to the quay and vice versa are facilitated becausethe transport structure hull is level with this quay.

1. A combination of a deck of an offshore fixed oil production platformand a structure for transporting, commissioning and decommissioning ofthe deck, the combination comprising: a supporting column operable to beanchored on the seabed when the platform is at a working location; thedeck supported by the supporting column when the platform is at theworking location; a hull constructed and configured for flotation andshaped to define an open space in the hull, the open space beingpositioned and configured to accommodate the supporting column thereinwhen the hull is positioned below the deck; lifting legs movably mountedon the hull, and operable to be lowered to rest upon the seabed; ajackhouse supported by the hull, a displacement apparatus being housedin the jackhouse and associated with each lifting leg, the displacementapparatus being operable to displace the lifting legs; a shuttle mountedon the hull and movable by the hull along the lifting legs, the shuttlebeing independent and separable from the jackhouse with the displacementapparatus and having a surface operable to engage a lower surface of thedeck so as to support the deck on the shuttle to position the deck onthe supporting column or remove the deck from the supporting column; anda locking device for locking the shuttle on the lifting legs.
 2. Thecombination as claimed in claim 1, wherein the displacement apparatuscomprise two opposing plates supported by the corresponding leg and eachplate having lateral faces, both of the lateral faces having a set ofteeth, at least two opposing units supported by the hull and eachopposing unit comprising a pinion driven rotationally and operating inconjunction with one of the sets of teeth.
 3. The combination as claimedin claim 1, wherein the shuttle is comprised of a respective independentunit for each of the legs, a guidance device respectively associatingthe respective one of the units with one of the legs, and lateralstabilization devices for stabilizing the deck in its support position.4. The combination as claimed in claim 3, wherein each shuttle unitincludes: a vertical branch including a guidance device comprising partof a locking device which cooperates with a respective one of the legsfor locking the shuttle unit in place on the associated leg; and ahorizontal branch for supporting the deck.
 5. The combination as claimedin claim 4, wherein the guidance device is comprised of two opposingflanges, each flange cooperating with the vertical branch, and a housingfor one of the plates on the associated leg.
 6. The combination asclaimed in claim 4, wherein the locking device comprises at least onecounterlocking rack, at least one actuation device displacing the rackbetween a retracted position and a locked position and in the lockedposition, the rack engaging one of the sets of teeth on thecorresponding lifting leg.
 7. The combination as claimed in claim 4,further comprising a first slideway on the jackhouse; each shuttleincludes at least one second slideway mounted on the vertical branch andoperable in conjunction with the first slideway mounted on thejackhouse.
 8. The combination as claimed in claim 4, wherein thehorizontal branch of each shuttle has a top face and includes elasticdeck bearing devices on the top face of the horizontal branch of theshuttle.
 9. The combination as claimed in claim 3, wherein thestabilization devices include at least one hydraulic or pneumatichorizontal displacement cylinder for each shuttle.
 10. The combinationas claimed in claim 1, wherein the hull includes rails for sliding thedeck horizontally between a position with the deck supported on theshuttle and a storage position of the deck outside the shuttle.
 11. Thecombination as claimed in claim 1, wherein the hull has an open partincluding a door movable from an open position for positioning thestructure beneath the deck and a closed position during transport of thedeck.
 12. The combination according to claim 1, wherein the shuttle ismovable upwardly by force transmitted from the hull as it is raised onthe legs, and is movable by the force of gravity as the hull is loweredon the legs.
 13. The combination as claimed in claim 1, wherein the hullis U-shaped, including opposed lateral sections and a link connectingthe lateral section for defining the open space in the hull.
 14. Thecombination as claimed in claim 1, wherein there are at least three ofthe lifting legs.
 15. A method for decommissioning and transporting adeck of a fixed oil platform from a production site to a deckdisassembly quay, the method comprising, in order: providing a combinedtransport and lifting structure, which includes a floating hull fittedwith hull lifting legs moveable vertically relative to the hull, and ashuttle which is movable relative to the lifting legs; positioning thecombined transport and lifting structure beneath the deck with theshuttle in a position also beneath the deck; lowering the lifting legsinto contact with the seabed; raising the hull and the shuttle on thelegs to bring the shuttle into contact with the deck; separating thedeck from a supporting column on which the deck is supported, furtherraising the shuttle on the legs to a position clear of the supportingcolumn; locking the shuttle in place on the lifting legs and laterallystabilizing the deck on the shuttle; lowering the hull relative to thelifting legs to bring the hull into flotation, and thereafter to raisethe legs off the seabed by reaction; displacing the combined transportand lifting structure laterally to move the deck away from thesupporting column; lowering the lifting legs again to the seabed;raising the hull on the lifting legs to bring the hull into contact withthe shuttle; unlocking the shuttle from the combined transport andlifting structure lifting legs so the shuttle is supported on the hull;lowering the hull with the shuttle and the deck supported thereon bymoving the hull relative to the lifting legs to bring the hull intoflotation, thereafter to raise the legs off the seabed by reaction;floating the combined transport and lifting structure with the decksupported thereon to the disassembly quay; lowering the lifting legs atthe quay to the seabed to stabilize the hull; releasing the deck fromthe combined transport and lifting structure; and offloading the deckonto the quay.
 16. A method according to claim 15, wherein the steps ofraising the shuttle are performed by raising the hull on the liftinglegs.
 17. A method according to claim 15, wherein the hull is U-shapedand there are at least three of the lifting legs.
 18. A method fortransporting and commissioning a deck for fixed oil platform from a quayto a production site comprising, in order: providing a transportstructure including a floating hull fitted with hull lifting legs and ashuttle displaceable along the legs; floating the transport structure toan operative position relative to the quay; lowering the lifting legsrelative to the hull to bring the legs into contact with the seabed tostabilize the hull; operatively linking the hull and the quay;transferring the deck from the quay to the transport structure such thatthe deck rests on the shuttle and laterally stabilizing the deck on theshuttle; raising the lifting legs to bring the hull into flotation;floating the transport structure to the production site; lowering thelifting legs relative to the hull into contact with the seabed; raisingthe hull and the shuttle on the legs with the deck supported on theshuttle; locking the shuttle in place onto the legs; lowering the hullto separate it from the shuttle and to bring the hull into flotation;raising the legs relative to the hull; positioning the deck above an oilplatform supporting column and again lowering the legs relative to thehull in contact with the seabed; raising the hull on the legs to bringthe hull into contact with the shuttle; unlocking the shuttle from thelegs; lowering the hull, the shuttle and the deck to place the deck onthe supporting column; securing the deck to the supporting column;lowering the hull with the shuttle resting thereon to bring the hullinto flotation; raising the legs relative to the hull; and removing thetransport structure from the production site.
 19. A method according toclaim 18, wherein the steps of raising the shuttle are performed byraising the hull on the lifting legs.
 20. The method according to claim18, wherein the hull is U-shaped and there are at least three of thelifting legs.